In general, photographic light-sensitive materials coated thereon with silver halide emulsions are subjected to various pressures. For example, ordinary negative films are bent when being charged into cartridges or cameras, or are pulled when being forwarded by each frame in a camera.
Sheet films, such as lithographic and medical X-ray films, are often bent or folded when handled directly with human hands.
In addition, every light-sensitive material is subjected to heavy pressures upon cutting and processing thereof.
When a light-sensitive material is subjected to pressures as mentioned above, silver halide grains contained therein are also subjected to pressures via gelatin (or vehicle or binder) or via a plastic film (or support). It is known that silver halide photographic light-sensitive materials experience changes in their photographic properties when silver halide grains contained therein are exposed to pressures, as reported in detail by K. B. Mather, J. Opt. Soc. Am., 38, 1054 (1948), P. Faelens and P. de Smet, Sci. et Ind. Photo., 25, No. 5, 178 (1954), P. Faelens, J. Phot. Sci., 2, 105 (1954), etc.
It has, therefore, been strongly desired to provide a photographic light-sensitive material which can be free from such changes in photographic properties caused by pressure.
In known methods to improve pressure characteristics of silver halide photographic light-sensitive materials, silver halide emulsions having a relatively small silver halide/gelatin ratio are employed, or a plasticizer, such as polymers and emulsions, is incorporated therein, so that pressures imposed thereon would not reach to silver halide grains. Examples of plasticizers so far proposed include heterocyclic compounds described in British Patent No. 738,618; alkylphthalates described in British Patent No. 738,637; alkylesters described in British Patent No. 738,639; polyhydric alcohols described in U.S. Pat. No. 2,960,404; carboxyalkyl celluloses described in U.S. Pat. No. 3,121,060; paraffins and salts of carboxylic acids described in Japanese Patent Application (OPI) No. 5017/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); and alkyl acrylates and organic acids described in Japanese Patent Publication No. 28086/78.
However, adequate effects could hardly be attainable by this means since plasticizers can be used only in limited amounts as the incorporation of plasticizers decreases mechanical strengths of emulsion layers, and the use of an increased amount of gelatin makes processing treatment slower.
In spite of the above facts, the means can be fairly effective and almost satisfactory pressure characteristics can be attained in the case of silver halide photographic light-sensitive materials employing emulsion layers containing spherical silver halide grains, i.e., hexahedral, octahedral or potato-shaped grains, which are less susceptible to distortions caused by external forces than flat grains having a large diameter/thickness ratio.
In general, tabular silver halide grains having a large diameter/thickness ratio are capable of forming high optical densities with smaller amounts of silver since they have a large covering area per unit when coated on a support. In addition, such silver halide grains also possess high abilities to capture incidented light, as well as excellent spectral sensitization properties. However, satisfactory pressure characteristics could hardly be obtained by the above-described means when tabular grains are employed since they are extremely susceptible to external forces due to their shapes.
For example, tabular silver halide grains formed by adding silver nitrate to a solution containing gelatin and potassium bromide and potassium iodide give a photographic emulsion which shows a considerable lowering in sensitivity when exposed to pressure. This sort of desensitization caused by pressure can be reduced by the use of pure silver bromide grains, or silver iodobromide grains having a completely uniform halogen composition throughout the grain which are produced by simultaneously adding both silver nitrate and halide solutions by the double jet method, without regeneration of nuclei. Such silver halides, however, are highly subject to fogs caused by pressure and, hence, disadvantageous in practical use.